1. Field of the Invention
The present invention relates to a fiber-drawing furnace for melting and drawing an optical fiber preform to make an optical fiber and its drawing method, and more particularly to a fiber-drawing furnace for drawing an optical fiber having low Polarization Mode Dispersion (PMD) by decreasing diameter variations and non-circularity of the optical fiber and a method for drawing an optical fiber using the furnace.
2. Description of the Related Art
Generally, an optical fiber is manufactured by drawing a transparent glass-sintered material, called an optical fiber preform, in a furnace at a high temperature. As well known in the art, a resistance furnace and an induction furnace are commonly used in drawing the optical fiber preform.
The furnace has a graphite core tube positioned vertically in a work place for receiving a preform supplied through an upper opening thereof. The graphite core tube is heated by resistance heating or induction heating and acts as a heating body. The graphite core tube is commonly heated up to 2,100˜2,300° C. in order to melt the preform and draw an optical fiber from the preform. Under such a high temperature, the graphite core tube may be easily oxidized. Thus, the core tube should be maintained in a non-oxidizing circumstance using helium, nitrogen, argon and the like for the prevention of oxidization.
In the furnace, SiO2 and C particles separated from the graphite core tube and the transparent glass preform react to thereby create SiC particles. If the silicon carbides or carbon particles adhere to the preform, a tensile strength at the adhered position is considerably reduced, which causes break of the optical fiber during a drawing process. In addition, a melt vapor generated from the preform can adhere to an inner wall of the graphite core tube and damage it.
To prevent such problems, an inert gas is relatively strongly blown into the furnace. The flow of an inert gas has a profound influence on the optical fiber formation. In other words, the inert gas introduced into the furnace creates a turbulence flow, particularly strongly at a neck-down zone where the optical fiber preform is melt and reduces its section abruptly. Such a flow of the gas becomes irregular in the circumferential direction of the preform. Therefore, a diameter of the preform near the neck-down zone varies irregularly, and a drawn optical fiber thus has an irregular diameter with an increase in the non-circularity.
An optical fiber drawn from the preform is cooled while moving down below a drawing device. At this time, a boundary layer of the inert gas formed on a surface of the optical fiber gives an influence on the cooling of the optical fiber. The inert gas introduced into the furnace cannot form a uniform laminar flow on the surface of the drawn optical fiber. This makes the optical fiber cooled irregularly in the circumferential direction, so increases variations in the diameter of the optical fiber.
As well known in the art, if a perfect circular symmetry of the optical fiber is not achieved, it may give rise to a birefringence during a signal transmission through the optical fiber, resulting in Polarization Mode Dispersion (PMD) which adversely affects high-speed optical transmission characteristics.
In order to provide a uniform laminar flow on an optical fiber preform, there is proposed a structure in which an upper end of the core tube for passage of the preform is sealed and an inert gas flows from an upper portion to a lower portion. German Patent DE-A 37 31 347 discloses a sealing structure in which a diaphragm ensures gastightness at the upper end of the core tube. This gastightness is achieved by the diaphragm which is provided with nozzles for injecting combustible gas that is ignited. However, this gastight device is too complex to install and also shows difficulty in accurately guiding the preform to a center of the furnace.
To decrease the diameter variations, there is also suggested a method for slowly cooling a drawn optical fiber until the optical fiber moves out of the furnace. In this slow cooling process, it is important to prevent a turbulent flow of the inert gas and maintain a uniform laminar flow in order to perform the uniform cooling. U.S. Pat. No. 5,637,130 issued to Nagayama et al. discloses a technique in which a cylindrical member is formed at a lower portion inside a furnace in order to cool the drawn optical fiber slowly and control a flow of the inert gas. However, sufficient cooling and uniform laminar flow cannot be obtained only by means of the cylindrical member. Particularly, impurity particles generated in the furnace cannot be fully discharged due to the cylindrical member, but stick to the preform or the optical fiber, causing frequent break.